Drilling, Tapping & Boring Guide

Ask three machinists to explain drilling, tapping, and boring, and you’ll usually get three overlapping but different answers depending on what they’re trying to achieve in the part. All three processes create or refine round features, but they serve different purposes. Choosing the wrong process can waste tooling resources, increase cycle time, or both.

Three Processes, One Family of Operations

Drilling, tapping, and boring all involve rotating a cutting tool, or the workpiece, around a common axis to produce a round feature. What separates them is intent: drilling creates a hole from solid material, tapping cuts a thread inside a hole that already exists, and boring enlarges or trues up a hole that’s already there. For example, drilling may be used to create a clearance hole for a bolt, tapping may be used to produce internal threads for a fastener, and boring may be used to improve the accuracy of a bearing housing.

Drilling: Creating the Initial Hole

A twist drill removes material to cut a round hole directly into solid stock. It’s fast and relatively low cost per hole. In many common applications, drilling is performed first to create an initial hole before tapping, boring, or reaming. The trade-off is accuracy — a drill can wander slightly as it enters the material, which is why precision applications add a follow-up step. Our earlier piece on drilling and tapping for precision holes covers how shops tighten up that initial accuracy.

Tapping: Cutting Internal Threads

A tap cuts a helical thread inside a pre-drilled pilot hole so a bolt or fastener can thread into it. A pilot hole is a smaller initial hole that guides a subsequent drilling, tapping, or machining operation. Tap drill size and thread engagement depth (how far the fastener’s thread sits inside the tapped hole) both need to match the fastener specification, and on CNC machines, rigid tapping synchronizes spindle rotation with axial feed so the thread pitch comes out exact rather than approximate. Our guide to high-performance CNC drilling and tapping goes deeper into getting this process right at production speed.

Boring: Refining Size, Roundness and Finish

Boring uses a single-point tool to enlarge and true up a hole that drilling has already created. It corrects position error and out-of-roundness (how far a hole deviates from a perfect circle) that a drill alone can leave behind, and it’s the process of choice whenever a hole needs a tight tolerance (only a small permitted variation from the specified size) or a fine surface finish (a smooth machined surface) — a bearing bore or a precision bushing fit, for example. Boring is done on a CNC lathe with a boring bar for round parts, or on a CNC milling machine with a boring head for prismatic parts, meaning blocky, box-like parts rather than round ones.

Comparison at a Glance

Drilling — purpose: create a hole from solid material. Typical tolerance: moderate. Best for: fast roughing operations and pilot holes.
Tapping — purpose: cut internal threads. Typical tolerance: set by thread class. Best for: fastener and assembly holes.
Boring — purpose: enlarge and true up an existing hole. Typical tolerance: tight, finishing-grade. Best for: bearing bores and precision fits.

How to Decide which Process or Sequence You Need

As a quick guide: need to create a new hole? Consider drilling. Need internal threads? Drill and tap. Need to improve an existing hole’s accuracy or size? Consider boring.

A hole for a bolt to pass through loosely: drilling alone is usually enough.
A hole a fastener needs to thread into: drill first, then tap.
A hole with a tight roundness or size tolerance, such as a bearing seat: drill first, then bore, sometimes followed by reaming.

Choosing the correct sequence is just as important as selecting the right process. Drilling usually comes first to remove bulk material, and tapping or boring is added afterward depending on what the finished hole actually needs to do.

Choosing the Right CNC Machine for the Job

A CNC lathe is the natural fit for round parts where drilling, tapping, and boring all happen in the same turret sequence — our guide to CNC lathe machine types breaks down which configuration suits which part family. For prismatic parts, a CNC milling machine handles all three operations in a single setup, which our guide to types of CNC milling machines covers in more detail.

Talk to Hasil Karya About Machines for Precision Hole-Making